1. Field of the Invention
The present invention is generally in the field of solid state imaging devices. More specifically, the invention is in the field of Complementary Metal Oxide Semiconductor (“CMOS”) imaging devices.
2. Related Art
CMOS image sensors are increasingly used in a variety of electronic systems and applications to provide motion picture or still picture capability. CMOS image sensors offer a number of technical advantages over alternative devices, such as low power consumption, high level of integration (and corresponding low total system cost), and flexible operating modes, among others.
A significant problem associated with conventional CMOS image sensors is the noise present in the images produced by such devices. For example, conventional CMOS image sensors suffer from noise associated with the process for resetting the photodiode in each pixel to a known potential after each exposure and in preparation for the next image. This noise, also referred to as “reset noise” or “KTC noise,” is often the single largest source of noise in camera systems employing conventional CMOS image sensors. The reset noise is proportional to the square root of KTC, where C is the capacitance of the sense node or photodiode/source follower gate combination in a typical active pixel sensor. Reset noise is typically 30 to 40 electrons one-sigma. Reducing the capacitance of the sense node can reduce reset noise, but results in a corresponding reduction in the total charge that can be collected and therefore undesirably reduces the overall dynamic range in the camera system.
Accordingly, there is a strong need in the art for a method of reducing reset noise in a CMOS image sensor.